Conventional MOS amplifier circuits, typically suffer from performance issues such as speed and power consumption. Speed in amplifier circuits is impaired as a result of parasitic capacitance elements and asymmetric operation. For amplifiers that provide output signals that transition from one power supply voltage to another, the operation is known as rail-to-rail operation. As power supply voltage values decrease, the voltage differential in rail-to-rail operation is important.
Conventional amplifiers typically use a plurality of gain stages to achieve a predetermined high gain. However, as a consequence it becomes more difficult to stabilize a closed loop system having a plurality of gain stages. It is desired to have only one dominant frequency pole in a closed loop system. However, a large number of frequency poles are created by the multiple gain stages. Non-dominant poles must be of a high frequency relative to the frequency of the dominant pole to avoid a reduction in the system's phase margin. Conventional methods used to compensate for the system include pushing the dominant pole to a low frequency or move the non-dominant poles to higher frequencies or both. These techniques require additional circuitry and thus circuit area and additional power to operate the circuitry and to push non-dominant pole to a higher frequency. Another technique is to pull the dominate frequency pole to a lower frequency. This can be achieved for example with passive circuitry, such as Miller compensation capacitors. Conventional amplifiers typically require a high power supply voltage to obtain both a high gain and a high frequency bandwidth or they do not have rail-to-rail output capability. Rail-to-rail capability is required for many applications as a greater span in voltage value for a signal permits a higher signal-to-noise ratio to be obtained. As power supply voltage values decrease, the small voltage span has only made this performance issue more critical.
Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention.